The aim of this research is to study individual differences in cognitive and personality development in relation to maturational rates at adolescence in the context of a neuropsychological model of hemispheric specialization of function. The study is intended to replicate previous findings and to extend them further in order to answer questions they raised. The data are being collected in a longitudinal design with an initial sampling of 90 pre-pubertal boys and 90 pre-pubertal girls from a middle-class community. They are to be retested two years later, when most of them are expected to show at least initial signs of puberty, thus making it possible to discriminate between early and late maturing individuals. The psychological battery includes measures of a) differential abilities; b) cognitive processes involved in the performance of tasks shown to be related to maturation rates; c) variation in hemispheric organization of function; and d) interpersonal behaviors. Measures of maturation include a) neuromotor development and b) physical development. Data analysis emphasizes relationships among measures as well as relationships between phychological function and rate of maturation prior to the onset of puberty and at puberty.